1. Field of the Invention
The present invention relates generally to optical devices, and in particular to brightness converters for optical devices.
2. Technical Background
The development of a single transverse mode laser when excited by a gain region that supports the lasing of multiple modes is useful for many different types of applications. The efficient coupling of the output of this laser radiation into single-mode waveguides for subsequent frequency conversion or similar frequency operation is still an area of development.
A single-mode waveguide such as a single-mode optical fiber is the favored transmission medium for telecommunications due to its high capacity and immunity to electrical noise. Single-mode fiber transmission is preferred over multimode fiber because higher-order modes exhibit much greater dispersion (typically the limiting factor for the fiber transmission distance at high data rates). Silica optical fiber is relatively inexpensive, and when fabricated as a single transverse mode fiber can efficiently transmit and otherwise couple signals in the 1550 nm band for many kilometers without amplification or regeneration.
If amplification is needed, erbium-doped fiber amplifiers (EDFAs) have been found quite effective in providing the required optical gain. Another example of an amplifier is a fiber with Raman gain. However, a need still exists for optical amplification and/or optical brightness conversion in many non-telecommunication applications, such as medical, printing, displays, “eye-safe” lasers pumping at a range of about 1480-1550 nm for providing a signal wavelength at 1600 nm, or defensive use of lasers as an optical device.
In search for a high powered laser source for telecommunications or non-telecommunication applications, the broad-area diode laser (BALD) remains the most efficient and least expensive pump source of a multimode pump light. Recent progress in semiconductor laser technology has led to creation of broad-area laser diodes with output powers of up to 16 W. The achievement of high total power in the face of the power density limitation at the facet due to the risk of optical damage requires the use of broad area gain sections with emission facet widths of hundreds of microns. Devices 100 μm (microns) wide with a slow-axis numerical aperture (NA) of less than 0.1 and output power of 4 Watts at 920 and 980 nm are now passing qualification testing for telecommunication applications. With proper coupling optics, the beam of such a laser diode can be focused into a spot as small as 30×5 μm with an NA of less than 0.35 in both transverse directions. The optical power density in such a spot is ˜3 MW/cm2, high enough to achieve gain transparency in 3-level laser systems.
One approach for utilizing inexpensive high-power broad-area pump lasers involves cladding-pumped, or double-clad fiber designs for the broad-area optical pump source. The advantages of cladding-pumped fiber lasers are well known. Such a device effectively serves as a brightness converter, converting a significant part of the multimode pump light into a single-mode output at a longer wavelength.
Cladding pumping can be employed to build a separate high-power single mode fiber pump laser. A source based on the pure three-level 978 nm Yb+3 transition has long been suggested as a pump for EDFAs because this wavelength is close to the desired pumping wavelength of 980 nm. However, the cladding-pumped technique has been determined in practice to be ineffective for pumping pure three-level lasers, such as the 980 nm transition of ytterbium, because of various laser design parameters that have to be satisfied.
One such laser design parameter is the selection or discrimination of the desired 3-level lasing mode over the harder-to-suppress 4-level mode. Practical double-clad amplifiers and lasers have been mostly limited to 4-level systems. Double-clad fiber lasers offer better performance for four-level lasing (where the lasing occurs in a transition between two excited states) than for the three-level one (where the lasing transition is between the excited and the ground state). For example, for the rare-earth ion, Ytterbium (Yb), the three-level transition is at 978 nm and competing higher-gain four-level transition is at about 1030-1100 nm.
In a double-clad laser, an outer cladding confines the pump light from a primary pump source in a large cross-sectional area multimode inner cladding. The much smaller cross-sectional area core is typically doped with at least one rare-earth ion, for example, neodymium or ytterbium, to provide lasing capability in a single-mode output signal. Typically, a neodymium-doped or ytterbium-doped double-clad fiber is pumped with one or several high-power broad-area diode lasers (at 800 nm or 915 nm) to produce a single transverse mode output (at the neodymium four-level transition of 1060 nm or the ytterbium four level transition of 1030-1120 nm, respectively). Thus, conventional double-clad arrangements facilitate pumping of the fiber using a multimode first cladding for accepting and transferring pump energy to a core along the length of the device. The double-clad laser output can be used to pump a cascaded Raman laser to convert the wavelength to around 1480 nm, which is suitable for pumping erbium.
The amount of pump light that can be coupled into a double-clad fiber inner cladding depends on the cladding size and NA. As is known, the “etendue” (numerical aperture multiplied by the aperture dimension or spot size) of the fiber should be equal to or greater than the etendue of the pump source for efficient coupling. The numerical aperture and spot size may be different in both axes so there may be an etendue in the x and y directions that must be maintained or exceeded.
Typically, a high numerical aperture NAclad, related to the difference in refractive index between the first and second cladding is desired. If there are two claddings instead of one, the index of the first cladding is nclad,1 and the index of the second cladding is nclad,2 such that NAclad=(nclad,12−nclad,22)1/2. In a well-known design, the first clad layer is made of glass and the second is made of plastic (fluorinated polymer) with a relatively low refractive index in order to increase the numerical aperture NAclad. Such plastic may not have the desired thermal stability for many applications, may delaminate from the first cladding, and may be susceptible to moisture damage.
In known 3-level double-clad host fibers, the laser cavity is formed by an input dielectric mirror which transmits the 920-nm pump band and reflects the desired 980-nm lasing band. For any input mirror of the fiber laser, it is desirable to reflect only the fundamental mode, at the laser wavelength, e.g., 978 nm, to form the input end of the optical cavity. A dielectric mirror at the end of the double-clad fiber or a weak fiber Bragg grating in the single-mode fiber, e.g., Corning® CS-980 fiber, coupled to the coupling end of the double-clad fiber serves as the output coupler for providing the output end of the cavity.
One of the primary technical challenges in a high power fiber laser is the formation of the input dielectric mirror across the multimode inner cladding of the double-clad fiber. Approaches include attaching a glass micro-sheet to the fiber endface or directly depositing a thin-film dielectric on the fiber endface, but both of these methods present their own technical hurdles.
A tapered fiber laser has also been proposed as an alternate optical pump. This two-stage laser has an optical pump source to provide a pump light at a pump wavelength and a first waveguide portion which when optically pumped at the pump wavelength is capable of lasing with an emission at a lasing wavelength. The first waveguide portion exhibits multi-transverse-mode behavior at the lasing wavelength. A second waveguide portion exhibiting a substantially single transverse mode behavior at the lasing wavelength is optically coupled together with the first waveguide portion. An optical cavity is defined by a multimode grating on the first waveguide portion and a single mode grating on the second waveguide portion and includes the first and second waveguide portions. As an example, the delta index or contrast index of the difference between the cladding refractive index and the multimode core refractive index could be between 0.04 to 0.06 for the low indexed germania (Ge) doped silicates multimode fibers of this approach.
As is known, the terminology “fiber Bragg grating” refers to a grating in which incident light is reflected back along the same fiber by a “short period” (a.k.a. Bragg) grating in the fiber and the fabrication of such gratings is known. Fiber Bragg gratings (FBGs) couple power from one mode to another provided that the propagation constants of the two modes satisfy the following grating equation:                                           β            1                    -                      β            2                          =                              2            ⁢            π                    Λ                                    Eq        .                                   ⁢                  (          1          )                    where β1 and β2 are the propagation constants of the two modes, Λ is the grating period in the fiber, and first order diffraction is assumed for simplicity. When a forward propagating mode reflects into the identical backwards propagating mode, the Bragg condition becomes λB=2neffΛ, where neff is the effective index of the mode (β=(2π/λ)neff) and lies between the core index ncore and the cladding index, nclad for guided modes (nclad<neff<ncore). Forward propagating modes may also reflect into other modes when mode orthogonality is no longer maintained, for example when UV induced index changes due to the FBG itself perturb the index profile sufficiently. The index profile needed depends on fiber geometry, cladding material, and the exact wavelengths for the particular application.
As with the double-clad fiber laser, to enable the maximum launch of optical power from the high power pump source into the laser cavity of either the double-clad fiber or the tapered fiber laser, the pump needs to have a large numerical aperture (NA) waveguide which is related to the index contrast. However, an increased index delta for providing power enhancement requires more design, testing, and manufacturing complexities to be first solved.
Even though many fiber solutions can be applied to other types of waveguides, such as a planar waveguide, the different trade-offs of planar processing also have to be considered before realizing the final solution. It is known that planar processing of optical integrated circuit enables the production of low cost devices.
Therefore there is a continued need to increase the power output of an optical device, while increasing the reliability and simplifying the packaging and manufacturing of the optical device.